Vehicle detector



Dec. 20, 1960 J. L. BARKER 2,965,893

VEHICLE DETECTOR Filed May 31, 1955 2 Sheets-Sheet 1 {'9' :1 P Low PASS Luau A55 oscnmroa H a L k AMPLIFIER HLTER AMPLIHER museum THRESHOLD To smmu.

V O EECTIFIER. AM UHER EELAY cgjriatg M as 57 1N VEN TOR. daw/Lflzwzee 2.) GJMQ W ATTORNEY Dec. 20, 1960 J. L. BARKER VEHICLE DETECTOR 2 Sheets-Sheet 2 Filed May 31, 1955 QPBMGO 1 M62358 #1 a INVEN TOR. z/m/fi/Aflmzae BY ATTORNEY United States Patent VEHICLE DETECTOR John L. Barker, Nor-walk, Conn., assignor to Eastern Industries, Incorporated, East Norwalk, Conm, a corporatlon of Delaware Filed May 31, 1955, Ser. No. 511,995

18 Claims. (Cl. 343-7) This invention relates to a traffic signal control employing ultra high frequency radio waves reflected from a moving vehicle in the roadway to actuate a traffic signal, and to a greatly improved road vehicle detection system employing the Doppler effect on such radio waves from such a moving vehicle.

More particularly the invention relates to a traflic signal control or traflic detection system deriving brief actuation pulses from vehicles passing a desired detection point in the roadway and distinguishing substantially from all other objects and influences ordinarily found associated with road traflic conditions.

It is a general object of the invention to provide n traffic signal control or trafiic detection system employing radio reflection, and of compact and low cost construction and highly reliable operation to'enable the practical control of traflic actuated signals by radar, and thus avoid the need for treadle switches, magnetic or other devices in or under the pavement of the roadway.

It is also an object of the invention to provide such a control or system which will provide a series of brief pulses from a close succession of spaced passing vehicles such that the number of pulses will indicate in substantial degree the number of vehicles passing, whereby this system may serve in traflic actuated signal and other control systems apportioning signal time or otherwise responding to the volume or rate of flow of traffic.

Other objects will be apparent from the following description and appended claims.

Trafiic actuated control systems employing means for detecting approaching vehicles on one or more of the roads or streets at a traffic intersection are already well known for controlling traflic signals.

Such traffic actuated control systems are widely recognized as of great benefit to the general public using the roadway in their ability to initiate a traffic signal time cycle in response to traffic requirements and in adjusting the length of the go periods of the respective roads at the intersection in accordance with the actual needs of trafiic as the latter varies from moment to moment as well as from the heavier peak traflic periods to the lighter off-peak times of day.

Such systems have in the past employed most largely treadle switches set in the surface of the roadway or magnetic detector elements set in or under the pavement of the roadway, in position to be actuated by approaching vehicles, ordinarily at a distance of 50 to 300 feet or more in advance of the intersection. At high speed traffic intersections such detector devices are sometimes set considerably more than 300 feet from the intersection. Examples of such tratlic actuated control systems may be found in the U.S. Patent 2,219,310 of H. A. Haugh, and in my U.S. Patent 2,265,991.

While such treadle switches and magnetic detectors have served quite well in the detection of approaching trafiic in connection with such systems, the installation of such devices is costly because of the need of breaking into the pavement and providing adequate foundation for 2,965,893 Patented Dec. 20, 1960 the installation of such devices, and also has a serious disadvantage in the need for blocking of the roadway temporarily during the installation period, which is not only hazardous to workers but also to the users of motor vehicles on the roadway and in many cases is a considerable traflic handicap where heavy trailic streets in cities are involved for example.

Furthermore, the more eflicient of such trafiic actuated control systems for the traffic signals take in account the traffic on the several parallel lanes on multi-lane approaches in very heavily travelled streets for example in order to make the maximum use of the intersection and to reduce the delays to traffic to a minimum, and thus these former pressure contact type treadle detectors and magnetic detectors have ordinarily been placed in adjacent traffic lanes on multi-lane streets and roadways on the approach to intersections in order to take account of the traffic in the several lanes, and it has been found to be important in this connection to have the detector devices respond to a large degree to individual cars or axles of cars.

It is also an advantage in traffic actuated signal systems to obtain an actuation pulse from vehicles of variable time length in accordance with the speed of the vehicles, such that the go signal time may be extended for such vehicles for a time period variable in accordance with the time needed for the vehicles to travel from the detector device to the signal, as in the U.S Patent 2,044,907 to F. G. Kelly for example.

In endeavoring to work out a practical radar vehicle detector to meet the particular needs and problems of road tralfic and of sufficiently low cost and small bulk to serve therefor, I have found that a remarkably simplified compact and effective such radar detecting device can be provided using the Doppler radio echo principle in a particular novel manner peculiar to road traffic conditions such that these radar detecting devices may be placed at a desired distance from the signal or road intersection, to respond substantially only to approaching vehicles, and to provide a series of brief output pulses from a succession of such vehicles sufhciently comparable to the pulses derived from the earlier pressure contact and magnetic detecting devices to operate trafiic actuated control systems reliably and efficiently. My new radar vehicle detector also provides an output actuation pulse whose time length varies in large degree in inverse proportion to the speed of the vehicle.

At the same time this new radar detector can be adjusted readily in position and sensitivity to enable its use to detect vehicles in two adjacent. traflic lanes if desired for example, and thus reduce the cost of traffic detection as compared with the earlier systems as well as providing a device which avoids entirely cutting into the pavement of the roadway. The latter point is of particular importance in connection with the frequent resurfacing or rebuilding of roadways.

This new radar detector is of such construction that it can readily be left in place while such work is proceeding on the roadway, is not subject to the wear and tear of traflic contact, and can readily be moved to a more desirable position or detection point along the roadway if traffic conditions change.

Referring now to the several drawings illustrating a preferred embodiment of the invention:

Fig. 1 shows a plan view of a section of roadway with a radar vehicle detector on the approach to a signal in accordance with one aspect of the invention.

Fig. 2 shows an enlarged elevation view of the radar detector antenna in relation to a roadway and a vehicle thereon.

Fig. 3 shows a block diagram of a preferred form of the invention. a

Fig. 4 shows a schematic circuit diagram of a preferred form of the invention.

Fig. 5 illustrates a preferred form of antenna unit and associated transmitter-receiver and electrical response circuit in a compact housing suitable for support by a mast arm over the road.

Referring to Fig. 1 in more detail a vehicle is shown schematically proceeding to the left of the figure, as indicated by the arrow, along a roadway whose edges are indicated at 11 and 12 and whose center line is indicated at 13. Supported over the roadway is an ultra high frequency radio antenna unit 15, illustrated as of the dipole type. This antenna unit is associated with a box or housing 16 which may support or contain the antenna unit and its associated electrical apparatus as shown in Figs. 3 and 4 for example.

The antenna unit is supported over the side of the roadway on which it is desired to detect approaching vehicles as for example over the right side of a roadway in which traffic drives to the right, as illustrated by the vehicle 10 approaching the signal S. The antenna unit on a two way roadway as illustrated is preferably mounted slightly nearer the side of the road than the center line of the road, by means of a supporting arm and connections 17 from a pole or post 18 and directs a beam of ultra-high frequency radio waves downward to produce an approximately elliptical pattern 19 on the same side only of the roadway and with its long axis substantially parallel to the roadway and extending from approximately directly under the antenna unit outward toward approaching traffic.

The edges of the roadway 11 and 12 are indicated as broken and extending on to the left at 11' and 12 to indicate some distance to the signal S which may indicate a traffic signal, as for example a trafiic right-of-way signal associated with an intersection of the road 1112 with another road not shown. The antenna detector unit 15-16 is illustrated as connected via line 17, line 23 to a pole 24 and associated signal controller 25 and thence along line 26 to control the signal S. The line 26 may also represent a support for signal S from the pole or post 24. It will be appreciated that under some circumstances the radar detector unit 1516 might be connected directly to control the signal and in other circumstances might be connected to the signal controller 25 which in turn would control the signal as a part of a traffic actu ated control system. In the latter case the system might involve the use of additional radar detector units for other or all of the approaches to an intersection for control of the traffic signal.

It will be appreciated that the antenna unit 15 and its associated electrical circuit apparatus of Figs. 3 and 4 might be mounted as one unit over the roadway with only its output connection extended to the controller 25 or signal S, or the antenna unit 15 alone might be mounted over the roadway and the remainder of its associated apparatus located at the side of the roadway on the pole 18 for example, or the electrical apparatus might be suitably divided between the pole and over the roadway to permit some adjustment of sensitivity for example from the side of the road.

The preferred relation of the antenna unit to the roadway for approaching trafiic is illustrated more fully in Fig. 2, to enable the detector unit according to the invention to derive a brief actuation pulse from an approaching vehicle, that is a vehicle on the approachside of the roadway, and to substantially disregard vehicles on the opposite side of the roadway. It will be noted that the antenna unit is mounted at a slight angle to the horizontal, which is preferably of the order of 15 degrees, so that its transmitting axis is at a corresponding slight angle from the vertical, and the antenna beam pattern between half power points is approximately degrees between the lines 27 and 28, the line 28 representing one edge of such beam pattern extending substantially vertically downward from the antenna to the roadway and the outer edge 27 extending outward along the roadway toward approaching vehicles at an angle of approximately 30 degrees, thus providing the approximately elliptical pattern 19 of the radiant wave energy within the one side of the roadway on which it is desired to detect the vehicles. -It will be appreciated that there is a diminishing amount of radiated wave energy somewhat outside of this pattern but the sensitivity of the electrical apparatus can be adjusted, in association with its particular frequency response feature described below, to substantially eliminate detection of vehicles on the opposite side of the road. The entire assembly is adjusted to respond ordinarily to a moving vehicle approximately at and somewhat beyond the position shown in Figs. 1 and 2 for example, although it will be understood that this may vary somewhat in actual practice with different types of vehicles and conditions, and in this connection a suitable height for mounting the antenna unit has been found to be about 16 feet above the roadway for example.

Referring now to Fig. 3 the ultra high frequency antenna unit 15 is illustrated at an angle to transmit a radio beam downward and reflected back from vehicle 10 along lines 20, the ultra high frequency energy for transmission from the antenna and the reflected energy both travelling on the common line 29, the transmitted energy being derived from the ultra high frequency oscillator-detector 30, and the received energy bearing the Doppler frequency shift effect from moving vehicle 10, being mixed in this oscillator-detector to provide a Doppler beat frequency output.

Since the radiant wave energy is directed downward at an angle toward the approaching vehicle in a substantially limited pattern, as indicated by the ellipse 19 in Figs. 1 and 2, the motion of the vehicle 10 will be at a considerable angle with respect to the radiant energy beam, and consequently the Doppler frequency shift effect produced on the radiant wave energy by the motion of the vehicle will be very much reduced in relation to the vehicle speed along the road. The Doppler frequency shift will be proportional to the road speed times the cosine of the angle of the latter speed direction with the shortest radial direction to the antenna at each instant of vehicle passage through the beam.

In accordance with the invention this is designed to derive a brief low frequency Doppler wave train as an actuating pulse as the vehicle passes through the beam. For vehicles moving at ordinary trafiic speeds this appears at the antenna and at the input side of the oscillatormixer at line 29 as a low frequency Doppler modulation of the transmitted ultra high frequency wave energy and appears as a low frequency output in the audio or subaudio range at the right side of the oscillator-mixer 30, with a transmitting frequency of 2455 megacycles per second for example. This particular low frequency output is distinguished by the filter combination 3-1--33 having the overall effect of a band pass filter, with this low frequency amplified and rectified to control the relay 37 via the threshold amplifier 36 which derives the substantial operating current for the relay at a desired threshold voltage level output of rectifier 35 characteristic of a desired minimum amplitude of input beat frequency.

In the preferred circuit arrangement the low Doppler beat frequency output from oscillator-detector 30 is applied first to the high-pass filter 31 which passes frequencies only above a very low level of the order of 20 cycles per second for example in order to distinguish from spurious very slowly moving reflections, before amplification. The output of the filter 31 is applied to the amplifier 32 and the output of the latter is applied to the low-pass filter 33 which cuts off frequencies substantially above the order of 40 cycles per second for example. The resulting low frequency band of the order of 20 cycles width centering on 30 cycles per second for example, is supplied to amplifier 34. The output of amplifier 34 is rectified to derive a direct current voltage at rectifier 35 which is supplied to the threshold amplifier 36 to control relay 37. The output of the relay is supplied directly to a signal or to a signal controller as desired.

Thus by employing an antenna pattern as described and illustrated the Doppler beat note from the moving vehicle is subject to a very large cosine angle factor which goes to essentially zero as the vehicle passes directly under the unit, and by selection of the Doppler beat note of low frequency value by means of filtering, it is possible to prevent vehicles approaching the detector at a distance from producing a Doppler shift of low enough beat frequency to get through the low-pass filter. However, as the vehicle approaches close to the antenna unit, the change in the cosine of the angle of the vehicles motion with respect to the angle of the radiant energy between the antenna and the vehicle, as this angle approaches ninety degrees in passing, causes each passing vehicle to produce a Doppler beat note in the low end of the frequency range such that even for high speed vehicles a very low frequency is derived as the vehicle approaches under the unit. This essentially limits the actuation range of the unit to a few feet in advance of the unit.

By using a constant transmitting frequency of the order of 2455 megacycles for example and using the entire antenna for continuous transmitting and receiving, it has been found possible to obtain a narrow antenna pattern as indicated with a compact eight element dipole in-phase broad side array with each dipole extending substantially horizontally transverse to the road for horizontal polarization for best reflection from vehicles, and to obtain the cosine factor modified Doppler beat frequency within the range of 20 cycles per second to 40 cycles per second for operation of the output relay only by a vehicle passing substantially under and immediately in advance of the antenna unit.

Angling the antenna unit about degrees so that most of the energy is directed on to the roadway on the approach side also assures that only one pulse is secured for each vehicle travelling under the detector.

With this arrangement also, for higher speed vehicles the cosine factor must be considerably lower in order for the Doppler beat note to fall below the high end of the low-pass filter, or in other words the high speed vehicle must be nearer to the unit within the beam pattern for actuation than the low speed vehicle. Thus with this design the effective range of the detector along the roadway is larger for a slow moving vehicle than a fast moving vehicle, which produces a wave train of longer time length for slower vehicles and a resulting output pulse from the relay which is in large degree inversely proportional to the speed of the vehicle.

Thus this design according to the invention enables individual pulses to be obtained from successive vehicles to a very high degree in a single lane of roadway, and although there is some slight loss in this relationship where the detector unit is used to cover two adjacent lanes of vehicles, the individual relationship of pulses to vehicles is maintained in the latter case to a large degree suitable for trafiic actuated control systems as previously described.

Referring now to Fig. 4 showing the schematic circuit diagram, this diagram is divided into sections, identified with corresponding parts of the block diagram of Fig. 3 by the broken lines 4-1, 42, 43, 44, 45, 46 and 47, for convenience of reference between Figs. 3 and 4.

The operation of the circuit of Fig. 4 is as follows. The antenna assembly 15 is shown schematically at the lower left of the drawing, the antenna assembly including two columns of four dipoles each side by side, from dipole 51 to dipole 54 on the right and from dipole 55 to dipole 58 on the left, all mounted on the reflector backing plate 60.

The antenna assembly is connected via coaxial line 61 to the coupling element 62 in the cavity of the ultra high frequency oscillator tube 65. This tube is a conventional co-planar triode of the lighthouse type 2C40 for example with a re-entrant type associated cavity 66.

This oscillator tube 65 generates the ultra high fre quency energy which is transmitted via line 61 to the antenna assembly and also receives the reflected wave energy as Doppler shifted in frequency, modified by the cosine factor with respect to a speed of the vehicle. Thus the generated and reflected wave energies are mixed in the oscillator-detector, and the Doppler beat frequency, that is the Doppler shift frequency difference as modified by the cosine factor, will appear as a periodic change in anode current on line 79, 78 and through resistor 77 and capacitor 81. This change of current produces a periodically varying voltage across resistor 77 in accordance with the low Doppler beat frequency, the oscillator also thus serving as a detector of this low frequency output.

Capacitor 83 serves to partially attenuate high frequency signals outside the desired range. The low Doppler beat frequency signal appearing with respect to ground across resistance 77 and capacitor 81 is applied via line 86 and blocking capacitor 87 and line 88 to the input of the first amplifier.

The variable resistor 75 connected at line 74 to a regulated D.C. positive power supply serves to control the average anode current supplied via resistors 76 and 77 to the anode circuit 71 of tube 65 thereby providing a means whereby the generated ultra high frequency out put of the tube may be controlled.

Bias for the oscillator tube is provided via grid resistor 68 to ground, with the cathode 63 connected to ground and to the cavity at 66. The heater circuit for this tube, and those for the other tubes in the circuit, are conventional and are not shown.

The Doppler beat frequency signal is partly filtered by the capacitor 87 in relation to resistance 91, the latter resistor being connected between ground and the lead 88 at the input to control grid 93 of tube 95 of the first amplifier stage. Thus the combination of capacitor 87 and resistor 91 serves as a high-pass filter to substantially attenuate frequencies appreciably below 20 cycles per second and pass all frequencies above this level, this combination therefore serving as the high-pass filter 31 in the block diagram of Fig. 3.

The tube sections 95 and and their associated circui-ts between the broken lines 43 and 44 comprise a conventional two stage resistance capacitor coupled amplifier having relatively high gain, with capacitor 92 providing some by-pass to ground from the anode circuit at 94, for high frequencies and noise.

The output of tube section 105 of this amplifier is coupled from its anode circuit by blocking capacitor 103 to the low-pass filter comprising capacitors 106, 109, 111, inductors 107 and 108, and load resistance 113. The inductors 108 and 107, connected in a series with the resistance 113 from the output side of the blocking capacitor 103 and ground, and the successive shunting capacitors 106, 109 and 111 of this low-pass filter provide substantial attenuation of frequencies above 40 cycles per second with severe attenuation of frequencies appreciably above 50 cycles per second.

Since the high-pass filter comprising coupling capacitor 87 and resistance 91 primarily sets the low limit of frequency which can be passed by the circuit as previously described, and this high-pass filter rapidly attenuates frequencies below 20 cycles and severely below 10 cycles, the low-pass filter between the broken lines 44, 45' and the high-pass filter to the left of the broken line 43 together provide in effect a band pass filter having a pass band of approximately 20 cycles, with the pass characteristic of the circuit peaked at approximately 30 cycles and with quite rapid attenuation above 40 cycles and below cycles, although the considerably reduced signals between 20 and 10 cycles and between and cycles may have some limited effect for strong input signals with high sensitivity adjustment.

It has been found that a band pass arrangement over this range has additional advantages since hum voltages corresponding to and 120 cycles from alternating current power supply and related sources are severely attenuated so as not to effect the circuit.

The output of the low-pass filter is obtained by adjustable tap on resistance 113 serving as a potentiometer adjustment of sensitivity of the entire circuit when it is connected into the input of the two stage amplifier illustrated between the broken lines 45 and 46 and associated with the tube sections and 125.

This is a conventional resistance capacitance coupled amplifier with the addition of series resistance 114 in the grid input to tube 115 and series resistance 123 in the grid input to tube 125 to prevent blocking of the amplifier by any excessive input signal strength.

The output of this amplifier is coupled from the anode circuit of tube 125 via coupling capacitor 128 to the dual diode rectifier detector 135, 145, the tube section providing a D.C. voltage across load resistor 144 in its cathode circuit of a positive value generally proportionate to the amplitude of the Doppler beat frequency signal as attenuated by the band pass filter arrangement. This rectifier is connected for doubler action so that the maximum voltage can be secured from its output.

The output of the rectifier-detector tube section 145, appearing across load resistance 144, is essentially a D.C. voltage and is connected via resistance 146 to the parallel connected grids of the dual tube 155, 165 serving as a threshold amplifier to supply operating current to the relay 37 above a desired voltage value of the D.C. voltage output of tube section 145. The cathodes of both tube sections and are connected in parallel via adjustable resistance 156 to ground and are also connected at junction 158 at the upper end of resistance 156 via resistance 157 to the B+ supply. The anodes of the tube sections 155 and 165 are also connected in parallel and via wire 166 through the coil of the relay 37, the other side of this coil being connected via wire 164 to an A.C. power supply indicated by plus in a circle. The coil of the relay is shunted by capacitor 168 to sustain the action of the relay on the A.C. power supply.

Thus the tube sections 155 and 165 are parallel connected and together are operated on fixed bias provided by the connection of the cathode to junction 158 on the potential divider provided by resistances 157 and 156 between the 3-}- supply and ground. This bias may be adjusted at the adjustable resistance 156 so that sufficient anode current to operate the relay will be provided at a desired D.C. voltage with respect to ground applied to the grids at junction 147, this voltage corresponding to the voltage output of the rectifier-detector 135, 145.

The sensitivity control 110, 113 provides means whereby slight changes in the amplifier gain characteristics and in the antenna and radio frequency power output, may be adjusted for to provide best operation in response to vehicle actuation.

Since the effective Zone laterally with respect to the roadway may be increased by either broadening the antenna beam or increasing the sensitivity of the unit, this sensitivity control also serves as a means for adjusting the lateral coverage.

The contacts 171, 172 are controlled by the coil of the relay 37 to close the output circuit on lines 173 and 174 for example to actuate a signal or a traffic actuated signal controller for example in response to passage of a vehicle on the approach side of the road at the vehicle detector antenna location for example.

Referring now to Fig. 5, there is illustrated a preferred mounting and housing arrangement for the antenna 15 and its associated transmitter-receiver and electrical response circuit of Figs. 3 and 4 for example.

In Fig. 5 the assembly is shown turned in the opposite direction to that in Fig. 1 and Fig. 2 to illustrate the slip-fitter or mounting bracket 137 on which the unit may be mounted on a mast arm for example. Thus the position of the unit in Fig. 5 is turned degrees from the position shown in Figs. 1 and 2, merely for the purpose of convenience of illustration in Fig. 5.

The overall housing of the antenna unit and its associated electrical apparatus is designated 16 in Fig. 5 to correspond with the box similarly designated in Fig. 1. This housing includes a main shell having an upper section 181 and a lower slightly recessed section 182, the recess being primarily for strengthening of the shell and for appearance. The lower section 183, housing the antenna unit 15, serves as a radome or protective cover for the antenna unit and is made of a fiberglass-polyester resin composition for example to permit the radio frequency energy to pass through it. The unit 16 is provided with a removable cover 184 at the top which may be attached to the shell by the catches 186.

The antenna unit in the lower section and some of the associated electrical apparatus in the upper section together with the lifting handle or bail 191 are shown in broken line outline. The entire inner assembly within the shell 181-182183 may be lifted out of the shell for inspection or replacement if desired.

A portion of a shelf member 189 in the middle section is also illustrated in broken line outline with an indentation cooperating with a keying projection 188 to assure that the antenna and its associated assembly inside the outer shell will be set in desired relationship in assembly or in connection with replacement after removal. The shelf can be provided with two notches to cooperate with the keying projection at 180 degrees spacing so that the entire internal unit can be turned at 180 degrees if desired to be used at the left side of an approach road from a mounting on a central esplanade on a divided roadway for example. Alternatively a single notch in the shelf may be provided with a removable keying projection which may be transferred from one 180 degree position to the other.

Although a preferred form of circuit for the invention has been illustrated and described above, it will be appre ciated by those skilled in the art that other forms of filtering may be provided to pass the desired narrow band of Doppler beat frequency characteristic of close passage of vehicles through the angled radio beam, as by a Wien bridge filter circuit for example, and the filtering may be applied at a different stage in the amplifying circuit, if desired in applying the teachings of the invention.

In providing a radar detection device according to the invention it has been found that the antenna assembly 15 may be of the order of 6 inches by 10 inches by 1% inches in size, and thus this unit and the entire assembly of Fig. 5 may be of sufiiciently compact size to serve as a practical radar detector unit for road trafiic, one which may be easily transported and readily and rigidly mounted over the roadway for reliable operation by desired moving vehicles and not by undesired vehicles or other extraneous moving objects or changing reflections.

It will be obvious that although the beam has been illustrated as angled toward approaching trafirc in the preferred form, it will be understood that this angle could be reversed 180 degrees with respect to such approaching vehicles so that the beam would be similarly angled along the same side of the roadway away from such approaching vehicles so as to detect such approaching vehicles as they pass beyond the antenna unit or in other words are departing from said antenna unit in passing through such oppositely angled beam.

It will be obvious to those skilled in the art that various other modifications may be made in the assembly of the apparatus and in the connections and circuit arrangements masses 9 and parts thereof without departing from the spiritof the invention within the scope of the appended claims.

I claim:

1. In a trafiic controlled signal system, a generator of ultra-high frequency waves having a wave length very much smaller than the width of ordinary road vehicles, means including a directional antenna disposed above a road and mounted with its transmitting axis at a slight angle from the vertical for transmitting a narrow divergent beam of said ultra-high frequency waves from said generator downward over a limited width of said road corresponding to the path of vehicular traflic approaching said signal and angled from the vertical for a limited distance along such path and for receiving back and supplying to said generator reflections of such waves as shifted in frequency by the Doppler effect of a vehicle passing through said angled beam, means responsive to a narrow band of frequency of said Doppler frequency shift at a low frequency to provide a brief output pulse as the vehicle passes through a part of said beam, and control means connected to said responsive means to be controlled by the output of the latter in response to such passage of a vehicle.

2. A combination as in claim 1 in which said responsive means includes means responding only to such a low frequency of such Doppler shift as to provide a brief output pulse only as the vehicle passes through the part of the beam nearing a maximum angle with respect to the line of travel of the vehicle along the road.

3. A combination as in claim 1 in which said responsive means includes means for deriving a brief output pulse varying in time length in substantial proportion inversely to the speed of vehicles passing along said road through said beam.

4. A combination as in claim 1, with said beam angled toward traffic approaching said signal.

5. In combination, means mounted over a roadway for transmitting a narrow beam of ultra-high frequency radio wave energy downward on to the roadway at a slight angle from vertical toward approaching vehicles and for receiving back such wave energy reflected from a moving vehicle in said beam as shifted in frequency by the Doppler effect of the component of vehicle speed toward the transmitting and receiving means caused by movement of the vehicle along the roadway through such angled beam and for mixing said transmitted and reflected wave energies and deriving the Doppler beat frequency therefrom, means for amplifying said beat frequency and means for filtering said beat frequency to derive a very low frequency component therefrom characteristic of passage of such vehicle through such angled beam, and means for controlling an output circuit in response to said low frequency amplified component.

6. In combination, means mounted over a roadway for transmitting a narrow beam of ultra-high frequency radio wave energy downward on to the roadway at a slight angle from vertical toward approaching vehicles and for receiving back such wave energy reflected from a moving vehicle in said beam and as shifted in frequency by the Doppler effect of the component of vehicle speed toward the transmitting and receiving means caused by movement of the vehicle along the roadway through such angled beam and for mixing said transmitted and reflected wave energies and deriving the Doppler beat frequency therefrom, means for amplifying said beat frequency and including means for filtering said heat frequency to derive a very low frequency component therefrom characteristic of passage of such vehicle. through such angled beam, means for rectifying said low frequency component to provide a direct current voltage in accordance with the amplitude of the low frequency component, means for providing a rapidly increasing current output above a desired threshold value of such direct current voltage, and relay means controlled by said current output.

7. A combination as in claim 6 and said filtering means deriving a narrow low frequency band characteristic of the Doppler shift component as the frequency of the latter passes through such band as the vehicle passes through said beam.

8. A combination as in claim 5 and said filtering means deriving a narrow low frequency band characteristic of the Doppler shift component as the frequency of the latter passes through such band as the vehicle passes through a part of said beam.

9. In combination, means mounted over a roadway for transmitting a narrow beam of ultra-high frequency radio wave energy downward on to the roadway at a slight angle from vertical toward approaching vehicles and for receiving 'back such wave energy reflected from a moving vehicle in said beam as shifted in frequency by the Doppler effect of the component of vehicle speed toward the transmitting and receiving means caused by movement of the vehicle along the roadway through such angled beam and for mixing said transmitted and reflected wave energies to derive a brief wave train of reducing Doppler beat frequency as the angle of the vehicles motion increases in passing through said beam, and means for filtering and amplifying only a narrow frequency band near the lower frequency end of said reducing frequency to provide a brief output pulse for the passing vehicle.

10. A combination as in claim 9 in which said output pulse varies in length in substantially inverse degree to the speed of said vehicle along the road.

11. A combination as in claim 9 in which said ultrahigh frequency is of the order of 2400 megacycles per second and said frequency band is of the order of 20 cycles per second at substantially 30 cycles per second.

12. In combination, an ultra-high frequency oscillator for generating constant frequency radio Waves, an antenna mounted over a road and connected to said oscillator for directing a narrow beam of such waves from said oscillator downward on to the roadway and at a slight angle with vertical toward approaching vehicles and for receiving such waves as reflected from a vehicle moving along the road through said beam and shifted in frequency in accordance with the Doppler effect of the radial component of such vehicle speed in said beam and for supplying said reflected wave energy to said oscillator, said oscillator serving to mix said generated and reflected waves and to detect the Doppler beat frequency therefrom, a high pass filter to attenuate extraneous low frequency components of the Doppler beat frequency level characteristic of passage of the vehicle out of the final part of said beam, an amplifier for the output of said high pass filter, a low pass filter for passing only low frequency components from said amplifier characteristic of said frequency shift as said Doppler component reduces toward zero in the passage of the vehicle through said beam, at second amplifier for the output of such low pass filter, a rectifier to derive a direct current voltage from the output of said second amplifier, a threshold response amplifier to provide a current output in response to said direct current voltage above a desired value of the latter, and a relay operated by said current output.

13. A combination as in claim 12, in which the angle of said beam with the vertical approximates 15 degrees.

14. A combination as in claim 12 in which said beam has a total angle of substantially 30 degrees between half power points and extending from substantially directly under said antenna outward for such 30 degrees toward approaching vehicles.

15. In combination, means mounted over a roadway for transmitting a narrow beam of ultra-high frequency radio wave energy downward on to the roadway at a slight angle from vertical and parallel with the direction of the roadway, with the major portion of the beam extending from directly under said .means in one direction along the path of vehicles passing along the roadway, and for receiving back such wave energy reflected from a moving vehicle in said beam as shifted in frequency by the Doppler effect of the component of vehicle speed with respect to the transmitting and receiving means caused by movement of the vehicle along the roadway through such angled beam and for mixing said transmitted and reflected wave energies and deriving the Doppler beat frequency therefrom, means for amplifying said beat frequency and including means for filtering said beat frequency to derive a very low frequency component therefrom characteristic of passage of such vehicle through such angled beam, and means for controlling an output circuit in response to said low frequency amplified component.

16. In combination, in a road vehicle detection system, a unitary'assembly for mounting over the road for detection of vehicles approaching closely thereunder along the road by radio echo from such vehicles, said unitary assembly including a housing, a directive antenna assembly for ultra-high frequency waves and removably mounted in the lower part of said housing at a slight angle with horizontal with its transmitting axis at a slight angle from the vertical to provide a beam angle along a line parallel with the direction of the roadway in a desired direction with respect to passing vehicles, means associated with said housing and said antenna assembly and including a projection on one of the last two mentioned elements and cooperating with a recessed part of the other of said last two mentioned elements for keying said antenna in such relationship when so mounted, and ultra-high frequency transmitter and receiver means connected with said antenna and mounted above the latter in said housing to be removable from said housing.

17. A combination as in claim 16, and in which said 12 antenna assembly and said transmitter and receiver means are assembled and removable as a unit from said housing, and in which said housing includes a removable cover.

18. In combination, means mounted over a roadway for transmitting a narrow beam of ultra-high frequency radio wave energy downward on to the roadway at a slight angle from vertical in one direction along the path of vehicles proceeding in a desired direction for detection in the roadway and for receiving back such wave energy reflected from a moving vehicle in said beam as shifted in frequency by the Doppler effect of the component of vehicle speed with respect to the transmitting and receiving means caused by movement of the vehicle along the roadway through such angled beam and for mixing said transmitted and reflected wave energies and deriving the Doppler beat frequency therefrom, means for amplifying said beat frequency and including means for filtering said beat frequency to derive a very low frequency component therefrom characteristic of passage of such vehicle through such angled beam, means for rectifying said low frequency component to provide a direct current voltage in accordance with the amplitude of the low frequency component, means for providing a rapidly increasing current output above a desired threshold value of such direct current voltage, and relay means controlled by said current output.

References Cited in the file of this patent UNITED STATES PATENTS 2,521,683 Barker Sept. 12, 1950 2,545,503 Tucker Mar. 20, 1951 2,620,470 Rather Dec. 2, 1952 2,622,140 Muller et a1 Dec. 16, 1952 2,785,395 Platzman Mar. 12, 1957 

